To transmit a turning force from a drive shaft to a driven shaft, a power transmission mechanism comprising sprockets and a chain has been conventionally used. In this power transmission mechanism, when a chain is extended over sprockets, the distance between drive and driven shafts is forcibly extended to apply initial tension to both forward and backward sides (i.e., reaches) of the chain so that looseness of the chain is eliminated, and even if the load torque of the driven shaft is varied to be positive and negative, the chain is not vibrated. Here, the forward side of the chain is the side on which the chain is moved in the direction from the driven shaft to the drive shaft during circulating movement of the chain, whereas the backward side is the side on which the chain is moved in the direction from the drive shaft to the driven shaft. The positive load torque is the load by which the forward side of the chain is tensioned, whereas the negative load is the load by which the backward side of the chain is tensioned.
However, the power transmission mechanism of the type as described has problems in that the assembling operation for applying initial tension is difficult, and elongation caused by wear occurs in both forward and backward sides of the chain as a result of use for a long period (the whole periphery of the chain elongates), and when the chain vibrates, the chain can rupture.
To cope with such problems as noted above, the chain is normally engaged by a tensioner so as to eliminate looseness in the chain. In this case, however, there involves another problem in that space for installing the tensioner is limited, and the tensioner increases the cost.
The above-described problems are believed solved by the present invention which provides a power transmission mechanism with two sets of chains, having first and second sets of sprockets, each pair of which is provided on a drive shaft and a driven shaft, a first chain extended over the first set of sprockets, a second chain extended over the second set of sprockets, and the two sprockets provided on at least one of said shafts being relatively changeable in rotational position thereof, whereby the forward side of one chain and the backward side of the other chain are tensioned.
Since the forward side of one chain and the backward side of the other chain are tensioned, even if positive and negative loads occur due to a variation of load torque, the positive load can be received by the chain of which a forward side is tensioned while the negative load can be received by the chain of which a backward side is tensioned. Accordingly, the tensioned side of either chain receives the load in response to the variation of the load torque whereby it is hard to vibrate the chain even if a variation of the load torque occurs, and even if the construction of a tensioner is simple or even if a tensioner is not used, a stabilized running of the chain can be obtained. Moreover, since the initial tension is applied merely to the tensioned side of each chain, the whole elongation due to wear of the chain decreases.